There is increasing interest in methods to reduce or capture CO2 from many different gaseous mixtures. CO2 is an undesired component that is present in many gas streams such as natural gas and effluent gases, and there is also much global interest in reducing CO2 emissions from combustion exhaust for the prevention of global warming. CO2 can be removed or captured by many means, such as physical or chemical absorption of the gas by a liquid or solid.
Currently, a common method of carbon dioxide capture from process streams in industrial complexes involves the use of aqueous solutions of alkanolamines, but usually on a small scale. The process has been used commercially since the early 1930s (see, for example, Kohl and Nielsen, Gas Purification, 5th Edition, Gulf Publishing, Houston Tex., 1997), and is based on the reaction of a weak base (alkanolamine) with a weak acid (CO2) to produce a water-soluble salt. This reaction is reversible, and the equilibrium is temperature dependent.
The use of alkanolamines as absorbents for CO2 (from power plant flue gases, for example) is somewhat disadvantaged in respect of the amount of energy needed to regenerate the CO2-rich solvent, the size of the CO2 capture plant, and the loss of alkanolamines to the environment. Among conventional alkanolamines, monoethanolamine (MEA) is considered an attractive solvent at low partial pressures of CO2 because it reacts at a rapid rate and the cost of the raw materials is low compared to that of secondary and tertiary amines. The costs of absorption processes using MEA are high, however, because of the high energy consumption in regeneration, and because of operation problems such as corrosion, solvent loss and solvent degradation. Furthermore, MEA can be loaded up to only 0.5 mol of CO2/mol of MEA, or 33 mol %, as a result of the stable carbonates formed.
Physical absorption systems have advantages over chemical absorption such as lower energy costs, but also have disadvantages such as solvent losses and low CO2 capacity. A need thus remains for systems and materials capable of providing low-cost, high-capacity methods of CO2 capture. Concurrently, there is also interest in methods to reduce or capture SO2 from many different gaseous mixtures. Ideally the same process and compounds could be used for both gases, with the capability to selectively release the gases upon demand.